To further show how my comment might be wrong: "In medicine and epidemiology, shift work is considered a risk factor for some health problems in some individuals, as disruption to circadian rhythms..."
I interpreted "shift work" to mean "night shift".
Although you’re probably right in the context of this study, I highly suspect when you sleep does affect things due to the evolutionary and biological significance of the circadian rhythm.
Shift workers certainly experience more variability too though. I did it when I was in college. Your sleep schedule for work is incompatible with your personal life. Want to go to your family’s for Thanksgiving? That’s right in the middle of when you’d normally be asleep. Etc.